The prior art relating to the preparation of write-on plastic surfaces is replete with numerous examples of diverse methods of physically or chemically treating a plastic surface to improve the ability of the plastic surface to accept ink from a pen or, alternatively, a printing ink (collectively and generally referred to in the prior art and herein as "ink"). The ability of an ink to be retained or adhere to a surface is commonly referred to as the "wetting" characteristic of the ink for that surface and reflects the extent of adhesion of the ink to the particular surface. This ability of the ink to wet the surface is also an indication of the writability of surface. Review of the prior art relating to write on thermoplastic surfaces is set forth hereinafter.
The prior art has disclosed both chemical and physical treatment processes for modification of the surface characteristics of thermoplastic materials. For example, U.S. Pat. No. 4,024,038 discloses chemical treatment of a plastic surface with a modification agent, e.g., a solvent, for the purpose of improving the adhesion characteristics of the surface of the plastic. Since chemical treatment is costly and difficult to implement in commercial manufacturing processes, the most commonly employed means for modifying, i.e. "treating", a plastic surface to improve its adhesion characteristics is the use of corona-discharge treatment. The use of corona-discharge for treating the surface of a plastic to improve the ability of an ink to adhere to the plastic surface, i.e., to improve the adhesion characteristics of the film, is well established in the prior art, as represented by the disclosures of U.S Pat. Nos.: 2,810,933; 2,844,731; 4,051,044; and 4,710,358. Use of corona-discharge treatment has typically addressed the treating conditions and not the compositional make-up of the plastic material being treated.
In addition to the patent literature, there is a significant body of non-patent, technical literature on corona-discharge treatment of plastic films. For example, the article "corona-discharge Treatment of Polyethylene Films I, Experimental Work and Physical Effects", Polymer engineering and Science, Vol. 18. No. 4 (March 1978) discusses the adhesion characteristics of corona-discharge treated polyethylene films and describes several testing procedures found useful in testing polyethylene films. Interestingly, the corona-discharge treated films tested by the article's author were limited to unmodified polyethylene films ("barefoot resins") and polyethylene films containing only low to medium concentrations of the slip additive commonly employed in the manufacture of polyethylene films. The article reported some interesting results. The presence of slip additive in the polyethylene film was noted to decrease the adhesion properties of the corona-discharge treated film, i.e., the treated film demonstrated reduced adhesion characteristics. Significantly higher corona-discharge treatments were required to increase the surface adhesion properties. Further, the article reported that the addition of slip additives drastically changed both the polar characteristics and dispersion characteristics of the film's surface energy. The article noted that corona-discharge treatment of the slip additive (commonly referred to as "slip agents") associated with the polyethylene films did not result in an improvement in the "Wipe" test or "Adhesion" test employed in the article for evaluating the surface adhesion characteristics of the films. These tests are commonly employed to evaluate the writability/printability of a corona-discharge treated film surface as a function of the surface adhesive properties of the film surface (often referred to as the "wetting" characteristics of the film surface by an ink).
Wetting characteristics of polyethylene films and the ability of such surfaces to have an ink adhere thereto have been discussed in the article entitled, "SURFACE CHARACTERIZATION OF CORONA TREATED POLYETHYLENE FILMS", ANTEC 85, pages 269-272 (1985). This article examined the printability of polyethylene film and the effect of corona-discharge surface treatment on the polyethylene film surface to improve ink wetting of the film surface, i.e., surface adhesion properties. The results reported by this article are interesting in many respects. The author reported that, "Surface roughness is a barrier to wetting and yields higher than expected [contact] angles" for unmodified low density polyethylene films ("barefoot resins"), i.e. a polyethylene film not containing slip or other processing additives. As expected, the author reported a clear relationship between the surface oxidation resulting from corona-discharge treatment and surface wetting of the evaluated barefoot polyethylene resins.
In another recent article entitled, "The Whole Story: Wettability, Corona Treaters and Compliance" CONVERTING MAGAZINE, September, (1986), the use of corona-discharge treaters is discussed in relation to in-line printing presses. The article is interesting in both its discussion of the effect of slip additives and use of water-based printing inks. The presence of slip additives on the surface of a modified film, i.e., slip additive-containing film, and the problems slip additives create in printing on the film surface are discussed. The author (Collins) reported interesting observations on the relationship of slip additives and the effect of corona-discharge treatment on slip additive-containing film. Several phenomena relating to the slip additives are discussed in the article, wherein Collins stated:
"It was observed that slip additives have a relatively low melt temperature. This means that subjecting the slip to excessive power levels would cause the slip additive to melt and become mobile, thereby obliterating the results of the treatment. PA1 Corona treatment is strictly a surface phenomenon, he explained, with depth of penetration measured in angstroms. Whereas the surface slip additives can be successfully treated, subsequent movement through melting can cause the effect to be lost. Melt temperature of the slip, according to Pillar tests, appears to be in the neighborhood of 180.degree. F."
After acknowledging the aforementioned phenomena the author discussed various alternatives to consider to resolve the problems encountered with slip additive-containing films. The thrust of the ensuing discussion related to adjustment of the corona-discharge treater and the role of the electrode and carrier roll in achieving optimum treatment results. (It is noted that the content of the article demonstrates that the author did not consider modification of the film composition as a means to improve the writability of the slip-additive-containing film.)
Although the patent and non-patent literature contain numerous references to both chemical and physical treatments of a plastic surface for the purpose of improving the adhesion characteristics of a film, e.g. the adherence of an ink to the plastic surface, there has been relatively little prior art discussion on the importance of the chemical composition of the plastic material being treated. Two patents discussing the composition of the plastic material as a means for affecting the adhesion characteristics of a plastic surface are discussed hereinafter.
An early reference to a plastic drawing material sheet is found in British Patent Specification No. 1,478,474. This British Patent discloses a plastic sheet material having a drawing layer which contains a crosslinked thermoplastic film-forming binder comprising more than 50% by weight of a polyvinyl butyral crosslinked by means of an etherified (polymethyl)-melamine in the presence of an acid catalyst, which polyvinyl butyral contained at least 18 per cent by weight of vinyl alcohol groups before crosslinking. The British patent discloses the drawing layers as suitable for use with graphite pencils and ink. In addition, the drawing layers can contain "toothing agents" selected from a wide range of particulate materials, including diatomaceous earth, glass powder, titanium dioxide, zinc oxide, kaolin and amorphous silica. The function of the "toothing agents" is somewhat unclear from the British patent, although the patentee states that a mixture of two toothing agents having different particle size and specific surface may be provided to improve the erasability of the drawing layer. One would assume that an improvement in erasability corresponds to a decrease in surface adhesion or a decrease in surface writability.
U.S. Pat. No. 4,801,487 discloses an imprintable sheet of a plastic comprising a carrier sheet and a coating layer applied on at least one layer of the carrier sheet for receiving an imprint. The coating layer to be imprinted consists of a firm polymer matrix and microporous finely divided solid particles. The patentee describes the non-imprintable carrier sheet as a non-polar polymer such as polyethylene or polypropylene. The firm polymer matrix of the coating layer is disclosed at column 3, lines 30-39 as consisting substantially of several polymers. These polymers are contrasted by the patentee with the polyethylene and polypropylene polymers employed for forming the non-imprintable carrier sheet. For example, the firm polymer matrix (the coating layer to be imprinted) may be selected from a large class of polymers, including polyacrylates, polymethacrylates, polyesters, polystyrenes, poly-1-3-dienes, polyamides, polyurethanes, polyvinyl butyral, polyvinyl acetate alkyde resins, urea resins and resins modified with natural materials such as starch and casein. The "microporous finely divided solid particles" are disclosed as absorbent solid particles and are discussed at column 3, lines 40-60. The extensive list of absorbent solid particles covers a wide range of solid materials having widely different chemical and physical properties, including white pigments and white or colorless fillers, such as aluminum oxide, barium sulfate, titanium dioxide, silicic acid, silicates, chalks, starch, melamine resins and/or formaldehyde resins. The function of the microporous finely divided solid particles cannot be surmised from this diverse random listing of materials. In fact, the patentee points out that the relevant criteria in selecting the microporous finely divided solid particles are that the particles be finely divided (having large specific surface area) and have a microporous internal structure. (See: column 3, lines 50-53) The specification and examples refer to the application of the firm polymer matrix as being a "coating" applied on the non-imprintable carrier sheet. The patentee actually refers to the firm polymer matrix coating as a "liquid primer". It is clear that the patentee's imprintable coating layer is related only to the firm polymer matrix with the microporous finely divided solid particles wherein the coating is applied as a liquid primer coating to a non-imprintable carrier sheet, e.g., a polyethylene or polypropylene film. The patentee discloses a corona-discharge treatment of the carrier sheet before the primer liquid is applied. It is clear that the corona-discharge treated carrier sheet is not considered to be a write-on surface, (i.e., the coating provides this function) but is merely treated to improve adhesion of the liquid primer to the carrier sheet.
The addition of calcium carbonate additives for polyolefin films has been reported in the prior art. For example, the use of OMYACARB.RTM. (OMYACARB.RTM. CaCO.sub.3 is sold by OMYA, 61 Main Street, Proctor, Vt. 05765) calcium carbonate mineral additive as a mineral additive in polyolefin films has been disclosed by the manufacturer to reduce the corona-discharge treatment levels by over 50 percent. Further, in sales literature on OMYACARB.RTM. calcium carbonate mineral additive, it is noted that "Early studies also indicate that the decay of Corona Treatment over time is minimized by the inclusion of a calcium carbonate mineral additive. It is believed that the microroughening effect on the film surface that is imparted by these mineral additives allows the reduction in Corona level and the blockage of decay. It is important to note also that this micro-roughening effect enhances film printability."
The above discussion is instructive in demonstrating that the prior art to date has not appreciated the complexity of the problems associated with write-on plastic surfaces as such relate to compositional changes in the formulation of the plastic material. These problems (such problems not heretofore appreciated by the prior art) are further complicated when the user is attempting to write on a plastic film surface manufactured in conjunction with a commercial bag manufacturing process. It has been observed herein that additional problems arise in commercial bag manufacturing processes owing to the numerous and variable manufacturing parameters of these processes. For example, it is common practice to employ a number of chemical processing aids to assist in commercial manufacturing processes for bags and film products. Such additives are typically provided as a "masterbatch" to the plastic material being processed. For example, a common additive in the masterbatch is a slip additive (commonly called a "slip agent"). The slip agent is added to improve machine handling of the plastic material, e.g., a plastic film, by increasing the slipperiness of the surface of the plastic so as to facilitate its ease of passage through the manufacturing equipment. The very slipperiness that is beneficial in the manufacture of the plastic material has been found to be detrimental to the end user's ability to write on the plastic film surface. The deterioration in the printability of the film as a result of film additives and/or oligomers is addressed in U.S. Pat. No. 4,832,772 wherein a physical surface "wiping" step in employed and is preferably employed with a preparatory solvent treatment of the surface. U.S. Pat. No. 4,832,772 discloses a process for wiping off a weak lay on the film surface containing oligomers and additives, although, surprisingly, no discussion of the nature of the "additives" is provided by the patentee.
As aforementioned, the use of a corona-discharge treatment of a plastic surface has been generally found to improve the writability or printability of slip agent-containing plastic materials by oxidizing the film surface and, if slip agent is present, "burning-off" a portion of the slip agent on the film surface. A discussion of the effect of corona-discharge treatment is set forth in the recent article by James F. Carley and P. Thomas Kitze, entitled, "Corona-Discharge Treatment of Polyethylene Films. I. Experimental Work and Physical Effects", POLYMER ENGINEERING AND SCIENCE, Vol. 18, No. 4 (March, 1978). The article is instructive in its disclosure that no changes in surface topography as a result of commercial corona-discharges are believed to contribute to practical adhesion phenomena and, further, that very high corona-discharge treatment was required to affect slip additives at the surface of polyethylene.
It has been found herein in accordance with the instant invention that this "burning off" effect and the associated improvement in surface adhesion characteristics is a short-lived surface effect when slip agent is present in the plastic composition or in close proximity to the plastic composition. This observation stems from the fact that corona-discharge is a surface treatment. The slip agent present in the bulk plastic composition is not affected by the corona-discharge treatment. It has been observed that this "bulk" slip agent in the plastic composition migrates to the corona-discharge treated surface as time passes, i.e., as the film ages, and the amount of slip agent at the surface increases with time. This "aging phenomenon" is related to the time the film has aged as well as the ambient conditions, e.g., temperature, under which the film has aged. Further, during many manufacturing processes, e.g., during the manufacturing of plastic bags and plastic films, the plastic bags and plastic films are stacked or placed on a roll in such a manner that film surface treated by corona-discharge contacts an untreated film surface having a greater amount of slip agent on the film surface that is present on the corona-discharge treated film surface. Once again, the net result of this contacting is an increase in amount of slip agent on the corona-discharge treated film surface as a result of the physical contacting of the two film surfaces.
The prior art has not appreciated and, accordingly, has not addressed the aforementioned problems associated with write-on plastic surfaces. Further, the unique problems associated with write-on surfaces associated with plastic bags has not been appreciated as to the unique problems associated with bag surfaces in physical contact with slip agents. As a result of this lack of appreciation, the prior art has not proposed effective means by which the write-on characteristics of plastic surfaces may be improved other than by simple mechanical or chemical surface treatment which are not effective for plastic surfaces that have undergone aging in the presence of slip agents.
The instant invention addresses these problems by providing a write-on composition suitable for corona-discharge (or other surface treatment providing a similar surface treating effect) having improved resistance to slip agent interference with the writability of the write-on film surface. Further, the instant invention addresses the unique problems associated with the commercial manufacture of plastic bags having a bag body formed from a plastic film containing at least one slip agent component and having at least one write-on surface on at least one wall of the bag body.